Electro-optical display devices are the object of considerable research efforts. Of the various display systems that have been developed, thin flat-panel display devices utilizing, for example, liquid crystal components are of particular commercial interest.
Compositions characterized as liquid crystals include a wide range of materials. The different electrical and optical properties exhibited by these liquid crystalline materials make possible a number of mechanisms for light modulation. Such mechanisms include phase transitions, dynamic scattering, and field effects, all of which are well known in the art.
Field effect devices are of particular utility. The effect that is commercially most significant at present is the rotation of polarized light by a twisted nematic liquid crystal alignment and the disappearance of this effect when an electric field is applied across the device. Twisted nematic liquid crystal devices typically comprise a suitable liquid crystal composition confined between two optically transmissive plates, the plates having transparent conductive films affixed to their surfaces facing one another in the device. The alignment of the surface layers of the liquid crystal in the "off" state (i.e., the "open" state in most liquid crystal displays) of the device is determined by the interaction of the liquid crystal composition with the confining surfaces of the display device. The orientation of the surface layers of the liquid crystal is propagated throughout the bulk of the composition.
To effect orientation of a confined liquid crystal, the internal surfaces of the conductive plates of a sandwich display device can be prepared by unidirectionally rubbing the surfaces prior to fabrication of the device. The liquid crystal molecules immediately adjacent each rubbed surface tend to orient themselves in the same direction as the rubbing. By arranging the opposing conducting plates with the axis of the rubbed surface at, for example, right angles to each other, the liquid crystal molecules at points intermediate the two plates will orient themselves to a degree which is a function of the distance from the two plates. Accordingly, in this example, the liquid crystal will align itself in a continuous spiral path that twists through the 90.degree. angle between the opposing plates.
If the light-rotating liquid crystal "sandwich" is mounted between two crossed light polarizer elements, polarized light will pass into the device (i.e., in its "open" state) and be rotated through a 90.degree. angle as it is transmitted through the twisted nematic crystal composition from one surface of the device to the other. Due to the 90.degree. light rotation effected by the twist of the liquid crystal, the polarized light will be set to pass through the second crossed polarizer mounted on the opposing side of the display.
When an electric field is applied across the liquid crystal composition between the two conductive plates, the twisted orientation of the liquid crystal is obliterated as the molecules align themselves with the applied field. As the liquid crystal is untwisted, polarized light entering the device through the first polarizer will no longer be rotated 90.degree. as it is transmitted through the liquid crystal. Therefore, the non-rotated light will be unable to pass through the second polarizer which is set correspondingly crossed to the first polarizer (i.e., the "closed" state). Selective application of voltages across discrete segments of the liquid crystal device can readily accomplish patterns of bright areas (no applied electric field) and dark areas (applied electric field).
In conventional devices employing LCD displays (e.g., wrist watches, calculators, personal digital assistants, cellular telephone displays, and laptop computers), backlighting and edge-lighting are oftentimes the greatest source of power drain. Thus, to reduce the energy requirements of such devices, it has been found that displays adequately viewable under ambient light can be provided by replacing conventional reflectors (or transflectors) with a reflective holographic diffuser, the reflective holographic diffuser comprising, for example, a holographic transmission diffuser and a reflection layer. See, U.S. Pat. No. 5,659,408 (Wenyon). In the use of a liquid crystal display incorporating such reflective holographic diffuser, polarized ambient light passing through a liquid crystal display element is transmitted through the holographic transmission diffuser, reflected by the reflection layer, then retransmitted as diffused light toward and through the liquid crystal display element, the diffuse light having gain within a predetermined viewing angle.
While the holographically illuminated liquid crystal displays disclosed in U.S. Pat. No. 5,659,408 accomplish good results in several applications, for applications where good achromaticity is of heightened importance, the diffuse illumination effected by the reflective holographic diffuser is limited by the intrinsic color of the polarizers (or other color absorbing elements) incorporated into the display. Perfectly neutral polarizers are uncommon, and more typically, a synthetic sheet polarizer will have an intrinsic spectral absorptance profile providing a distinctly perceptible hue, for example, "bluish", "reddish", etc. As a consequence, despite the use of a reflective holographic diffuser made to effect an achromatic spectral output, transit of ambient illumination through the liquid crystal display element will result in tainting the light perceived by a viewer. Accordingly, there is a need for means for compensating the spectral qualities intrinsic to liquid crystal display elements that compromise, undermine, or otherwise degrade the achromatic illumination diffusely reflected by said reflective holographic diffuser.